You can't have an inflatable spacecraft without an inflatable structure. That structure -- like the frame of a house -- is what gives the heat shield its shape and strength. But unlike the frame of a house, an inflatable structure only gets stiff after being pumped up with gas. That way it can start out packed in a relatively small bag on board a rocket, launch from Earth and grow to become larger in diameter than the rocket that carries it.
NASA engineers are studying different designs and materials. The inflatable structure has to be made of incredibly strong yet flexible fabric so it can keep its shape and withstand the force of rushing through an atmosphere at as much as 25,000 miles per hour (about 42,300 kilometers per hour).
The current inflatable design comes from a partnership with a private company, HDT Global. It's a series of inflated rings made of braided Kevlar -- the same material as bulletproof vests -- that are stacked together. Each is lined with silicon on the inside. The scientific name for the donut-shaped ring is a torus. The Kevlar gives each torus its strength, while the silicon liner keeps the compressed gas inside -- sort of like a bicycle tube and tire. Kevlar straps keep the rings attached to each other and to the payload hardware.
During launch the uninflated stacked rings are packed inside a bag, much like clothes vacuum packed inside a duffle bag or a closed umbrella. Before the spacecraft is getting ready to enter the atmosphere a compressed gas system pumps up the volume of the inflatable structure. That's one of a HIAD's unique features. Unlike traditional spacecraft that use rigid materials, such as space shuttle tiles or Mars capsule aeroshells, a Hypersonic Inflatable Aerodynamic Decelerator can start small and grow in size to carry more weight to distant worlds.